Many electric power generating stations utilize nuclear boiling water reactors. In these stations. a reactor core heats water to generate a two-phase steam/water mixture. The steam/water mixture is conveyed to a steam separator assembly underlying a steam dome in the reactor. The steam separator assembly classifies the steam from water in the two-phase mixture.
The steam is provided to a turbine-generator which produces electric power. Electric power is transferred from the turbine-generator to an electric power grid, and from the grid is distributed to the entire utility system.
The spent steam from the turbine-generator is generally ducted to a condenser. The condenser converts the spent steam into condensate. A feedwater injection system returns the condensate to the nuclear boiling water reactor as feedwater and as reactor coolant. In many nuclear boiling water reactors, the water which is separated from the steam/water mixture by the steam separator assembly provides the remainder of the reactor coolant. The reactor coolant inventory initially within the reactor, or resupplied to the reactor under various episodes, provides heat removal from the fuel rods which thus maintains the fuel cladding material at acceptable temperatures at all times.
Various events, such as a fault on the utility transmission lines, can cause the power station to become isolated from the main power grid. When the station becomes isolated, the turbine-generator no longer drives a load.
Many power generating plants employ steam stop valves to prevent steam from reaching the turbine when the turbine-generator becomes electrically isolated. These stop valves are coupled between the reactor and the turbine-generator and will rapidly close when the turbine-generator is isolated from the load.
Simultaneously with closure of the steam stop valves, station control procedures will trigger a reactor SCRAM. The reactor SCRAM scenario causes control rods to be inserted into the fuel assemblies to produce a subcritical core and, thus, to terminate the nuclear fission process. Total shutdown of the nuclear reactor occurs within a few seconds.
Notwithstanding that the reactor is shutdown following such a transient, a problem arises in that large quantities of steam were generated in the few seconds between stop valve closure and core fission cessation. This generated steam becomes added to the normal steam inventory, present both in the reactor steam dome and in the main steam lines connecting the reactor vessel to the turbine-generator, causing a further increase in steam pressure.
In addition, even though the reactor is shutdown, heat is continuously generated because of the radioactive decay of fission by-products in the fuel rods. This continuous addition of heat to the water in the reactor causes steam to be continuously generated and further increases the pressure within the isolated reactor vessel. This problem of continued decay heat generation is present not only immediately following isolation of the reactor from the turbine generator, but also throughout long time periods following shutdown of the nuclear reactor.
Various isolation cooling systems have been employed for dealing with this excess steam. Some stations are provided with bypass systems which use fast acting valves to shunt the steam from the reactor to a main condenser. These systems require substantial extra condenser heat removal capacity, bypass valves, and pipelines, which all together are very expensive.
Since, on occasion, steam from the reactor may be radioactive, it is necessary to avoid venting the reactor primary steam directly to the power station environment. Thus direct venting of reactor primary steam to the atmosphere is not a feasible alternative for an isolation cooling system.
Some isolation cooling systems relieve the excess steam intermittently through steam relief valves installed on a multiplicity of pipelines coupling the reactor to underwater discharge points within a large pool of water. One such commonly used arrangement employs the suppression pool located within the containment building of nuclear boiling water reactor power stations as the receiver pool for these steam discharges. These isolation cooling systems utilize various types of coolant makeup systems to replenish the reactor coolant lost by venting. These systems are expensive, however, because of the size and multiplicity of the relief valve discharge lines, their penetrations through containment structures, their end-termination steam quenchers, and the coolant makeup systems that provide coolant replenishment.
Still other isolation cooling systems shunt the excess steam to one or more isolation condensers during a reactor isolation condition. The isolation condenser is typically configured as a large elongated cylindrical tank. The isolation condenser is usually located at an elevation higher than the reactor steam dome, such as on an upper floor within the reactor building.
The supply side of the isolation condenser is typically connected by pipes to the reactor steam dome. A piping network returns cooled, condensed steam back to the reactor as coolant.
The isolation condenser requires a large initial coolant inventory for cooling the hot reactor steam, and heat exchange surfaces through which the steam flows. To minimize the need for cleaning the exchange surfaces, demineralized water is normally used as the water coolant in the isolation condenser.
Upon the occurrence of an isolation transient, steam is shunted from the reactor steam dome to the supply side of the isolation condenser. The hot steam, typically around 545.degree. F., transfers its heat through the heat exchange surfaces to the surrounding water coolant. During such isolation cooling, heat transfer rates are sufficiently high to boil the water coolant. The steam generated by boiling may be safely ducted to the outside atmosphere since the demineralized water is nonradioactive.
Conventional power stations have employed an additional long term heat removal system called a "shutdown cooling system" to reduce the temperature in the reactor down to around 125.degree. F. Conventional shutdown cooling systems usually rely on dedicated heat exchangers and a separate piping and valving network distinct from the isolation cooling system. Generally, these systems circulate hot reactor coolant through the shell side of the shutdown cooling heat exchanger and circulate cooler intermediate coolant through the tube side of the heat exchangers; however, a number of design alternatives are available depending on the specific application and power station.
Unfortunately, conventional shutdown cooling systems are capital intensive, and therefore costly, in several notable respects.
First, these systems require separate piping, valving, control, and heat transfer equipment above that required for isolation cooling, and therefore represent a significant plant capital cost.
In addition, these systems are large. The heat transfer challenge for these systems is relatively great because of the small temperature differential between the target tmperature, i.e., 125.degree. F., and the temperature of available coolant. i.e., 95.degree. F. To compensate for this low temperature differential, shutdown heat exchangers must use substantial amounts of heat transfer surface area and coolant. For example, medium power rated boiling water reactors having access to 80.degree. F. coolant, require a heat exchanger 5 feet in diameter and 30 feet in length. Larger power rated reactors using warmer coolant employ heat exchangers up to 6 or 7 feet in diameter with dual shells which are each up to 40 to 50 feet in length.
Shutdown heat exchangers are therefore large and costly. In addition, these heat exchangers are normally housed in the reactor building and require a long tube pull space to facilitate maintenance. Thus, the size of the heat exchangers can often be a controlling parameter, significantly influencing the physical design of the reactor building.
Another problem is created if the coolant in the heat exchanger is not demineralized water. If not demineralized, then the coolant often generates deposits or fouling conditions which increase maintenance problems. On the other hand, the reactor coolant is clean. Since it is more difficult to clean the outside surface of the heat-exchanger tubes than their insides, it is desirable that coolant from the reactor be piped to the shell side of the heat exchangers to minimize the frequency of external cleaning and maintenance. Since the reactor coolant is highly pressurized, this requires that the heat exchanger shell carry a design rating up to several hundred psig. As a result, the heat exchanger must have relatively thick walls or some other expensive shell configuration.
Some shutdown cooling systems cool the reactor coolant using clean water circulated within a system known as a reactor closed cooling water system--often termed a "RCW system". The RCW water is in turn cooled at a buffer heat exchanger using ordinary (often dirty) secondary coolant--such as river water.
One benefit of this technique is that RCW water is normally demineralized and poses less maintenance problem than the dirty secondary water coolant. However, when two heat exchangers are used, the driving temperature differential for each is cut in half. Thus, the size of each heat exchanger must be approximately doubled to meet the shutdown heat exchange duty. Thus, power stations that use intermediate buffer heat exchangers for shutdown cooling require even larger and more expensive exchangers and reactor building space.
Recently designed nuclear boiling water reactors have begun to integrate some of the various required emergency cooling functions. For example, one known boiling water reactor seeks to combine the isolation condenser with a gravity-driven cooling system. This gravity-driven cooling system performs emergency core cooling for the nuclear reactor core during a loss-of-coolant accident (LOCA). The gravity-driven cooling system employs a suppression pool which is elevated relative to the reactor core and filled with a coolant inventory during normal operation of the reactor. Following a loss-of-coolant accident, the reactor pressure vessel is depressurized. When the pressure inside the reactor falls below the gravitational head of the elevated pool of coolant, the coolant flows into the reactor pressure vessel to cool the reactor core.
The heat exchange surfaces that are normally housed in the isolation condenser shell are located directly in the suppression pool in one known advanced reactor design. One advantage of this arrangement is that it eliminates the isolation condenser shell. However, this arrangement locates the heat exchange surfaces under water in the suppression pool. Not only is the suppression pool itself remote, but the underwater location creates additional maintenance problems for the heat exchange surfaces.
Further, the suppression pool coolant is not normally intended to be vented to atmosphere because its contents may be slightly radioactive. Thus, the suppression pool with its isolation condenser heat transfer surfaces must be designed to prevent undue level swell and boiloff of the suppression pool coolant when used to remove heat from the steam. In addition, the heat exchange surfaces must be arranged in sophisticated configurations to optimize the heat removal process.